I was initially planning to post this on our company blog, but it took too much time to get comments from Sennheiser, so I shelved the article. Some time in January my lab guys started complaining about getting these strange Sennheiser HD650's. This headphone is the most popular among our individual calibration clients, so our lab blows through them rather intensely. The stuff below is me discussing what's found in the measurements. The Data First of all, let’s take a look at the data and see what we can see. The below graphs are averaged measurements taken from randomly picked HD650 headphones from Q2 2018 and “strange” looking HD650’s from Q4 of the same year when they started popping up. It should be rather evident that the changes aren’t night and day. The new style HD650’s aren’t a different headphone. The biggest difference is in the upper mids and treble. Overall the 2 - 10 kHz region displays a sensitivity reduction by -3dB and a pronounced dip has appeared at 7 kHz. Our engineering group unanimously agreed that the difference is indeed audible - the “strange” HD650’s sound darker and the treble dip can be troublesome for precise work on say cymbals. It also seems that the consistency for which we’ve constantly praised Sennheiser has gone down. Here are 5 measurements that were averaged in the above graph from randomly picked samples. The consistency is spectacular right up to treble. Now let’s look at what the consistency is between the “strange” specimens. Overall it’s similar with midrange getting more spread. Here's the kicker though - we're seeing that both variants of the HD650 can be bought right now, hence the real scatter in frequency response is rather large. We actually waited a few months, before saying anything to see if the change becomes permanent or goes away completely, too bad that it's here to stay mixed in with the old tuning variants. So, what now? Sample to sample variance is a reality for all headphones. Here at Sonarworks we've measured close to 6000 headphones and with many of them being of the same model we've have a unique insight into this fact of life. There's variance between different samples of the same headphone model and there is usually a difference between the tonal response of each channel, because drivers aren't ideal and differ a little from one another. Traditionally companies have got around the natural variance of products by implementing better manufacturing and quality assurance. Of course it costs money, because by buying a pair of headphones you're also paying for the drivers that didn't make the cut. Our customers who buy individually calibrated headphones should fret much, because calibration gets rid of any tonal aberrations. If you're just looking to get a pair of HD650's, then it's a bit harder. We took apart one of the “strange” HD650's and there's no way of visually discerning between both variants. Hence it's no use begging the vendors to guarantee that you're getting the one you want. The only way to be sure would be to measure. Now the great reveal... Initially I was convinced that this has something to do with Axel Grell leaving the company - you know, Sennheiser falling from grace and all that. Only today I finally solved it! As it turns out, Sennheiser has moved HD650 manufacturing from Ireland to Romania. It's likely that the discrepancies we were seeing were them adjusting to the new manufacturing process. Funnily enough, the HD600's that come to us are still made in Ireland, which may have to do with the fact that they don't sell as fast. So yeah, the new Romanian HD650's measure almost identical to the old Irish ones, it's all good.